1. Field
The following description relates to a PWM signal generating circuit for a DC-DC converter using a dimming signal and an LED driving circuit using the same, and, more particularly, to a PWM signal generating circuit for a DC-DC converter which may reduce the ripple of output voltage by prolonging a time during which an input dimming signal is turned, on and a Light Emitting Diode (LED) driving circuit using the same.
2. Description of the Related Art
A Liquid Crystal Display (LCD) type of display apparatus has become popular and widely used due to it having many advantages compared to other types of display apparatuses. A few of the advantages of the LCD over the other types of display apparatuses are a slim thickness, light weight, low power consumption, and low driving voltage. However, an LCD does not emit light by itself, and thus a backlight is used in order to supply light to an LCD panel.
Conventionally, a Cold Cathode Fluorescent Lamp (CCFL) and a Light Emitting Diode (LED) are two elements that are used as the aforementioned backlight source of an LCD. However, since the CCFL uses mercury, it may cause environmental pollution, as well as having a low speed response. In addition, the CCFL has other disadvantages such as poor color reproduction and not being appropriate to realize an LCD panel which has a high density, small size, and thin shape.
On the other hand, the LED does not use a material harmful to the environment, and is capable of performing with faster response time as compared to the CCFL. In addition, the LED has the advantages of good color reproduction and an ability to adjust brightness and color temperature arbitrarily by adjusting the amount of light of red, green, and blue LEDs. In addition, the LED is suitable for realizing an LCD panel of a high density, small size, and thin shape. Therefore, the LED has become more widely used as a light source for a backlight of an LCD panel in recent years.
As such, in conventional applications, if an LCD backlight using an LED employs a plurality of LEDs connected in a serial fashion, a driving circuit to provide constant current to the LEDs is required, and a dimming circuit to adjust brightness and color temperature arbitrarily, or perform temperature compensation, is required.
In more detail, different dimming methods used in an LCD backlight using an LED include an analog dimming method and digital dimming methods. The analog dimming method includes adjusting the brightness of the LED by adjusting an amount of electric current supplied to each of the LEDs. A pulse width modulation (PWM) dimming method, which is one of the digital dimming methods, includes adjusting the brightness of the LED by adjusting a ratio of on-off time of each LED. For example, if a PWM signal having an on-off time ratio of 4:1 is provided to each LED, the brightness of the LED reaches 80% of maximum brightness.
In order to adjust the brightness of the LED in the above-described digital dimming method, a clock signal of a DC-DC converter for adjusting power of the LED and a dimming signal for adjusting an amount of electric current in the LED are separately provided. In general, the frequency of the clock signal of the DC-DC converter is relatively high, and the frequency of the dimming signal is relatively low, and the clock signal of the DC-DC converter and the dimming signal are not synchronized with each other. Further, an on-period of the clock signal and the dimming signal are typically very short relative to the overall period of the respective signals.
In this case, if an on-period of the dimming signal is shorter than the period of the clock signal, the DC-DC converter may not be boosted at a time at which a specific dimming signal is turned on. If the DC-DC converter is not boosted, it is difficult to maintain a desired output voltage to drive the LED. This will be explained in more detail with reference to FIG. 8.
FIG. 8 illustrates waveforms occurring in an operation of generating a PWM signal based on a dimming signal in the prior art. Referring to FIG. 8, ‘PWMI’ indicates a dimming signal input from outside (PAD) and ‘B_CLK’ indicates a boosting clock signal. In addition, ‘PWM_OUT’ indicates a PWM signal supplied to the gate of the power transistor of the DC-DC converter, and ‘VOUT’ indicates output voltage in a case in which the above-described PWM-OUT is supplied to the DC-DC converter.
Referring to FIG. 8, in the case in which an on-period of the dimming signal (PWMI) is shorter than the period of the clock signal (B-CLK) of the DC-DC converter, the PWM signal (PWM_OUT) supplied to the DC-DC converter may not be generated in response to the dimming signal (PWMI) being turned on. If no PWM signal is generated during one period of the dimming signal (PWMI), the DC-DC converter cannot maintain a stable output voltage.